Metal plating



United States Patent Ofilice 3,049,437 Patented Aug. 14, 1962 3,049,437 METAL PLATING Melvin F. Rejdak, deceased, late of Cleveland, Ohio, by Hattie Wagner Rejdak, administratrix, Cleveland, Ohio, assignor to Erico Products Inc., a corporation of Ohio No Drawing. Filed Sept. 2, 1959, Ser. No. 837,569 16 Claims. (Cl. 117-31) This invention relates as indicated to metal plating, and more particularly to a method of plating amalgams of tin and certain other metals which is particularly adapted to use in the field where it may be desired to prepare surfaces for brazing and the like.

Metals such as tin may be plated on other metal surfaces by a variety of well-known methods including the common hot dip method, electroplating, and the employment of a chemical displacement reaction. None of these methods, however, is conveniently adapted for employment in the field where it may be desired to plate relatively small surface areas preparatory to brazing, for example, or to protect such surfaces against corrosion.

It is accordingly a principal object of my invention to provide a method of plating certain metals, particularly tin, which may be conveniently accomplished in the field.

Another object is to provide such method which does not require the application of external heat or electricity, or require employment of a chemical displacement reaction.

A further object is to provide such method which will produce a surface coating of substantial thickness and impermeability and capable, in the case of such metals as tin, of being burnished.

A still further object is to provide such method in which an amalgam of meta-ls is alloyed with the surface of the metal to be plated.

Other objects of the invention will appear as the description proceeds.

To the accomplishment of the foregoing and related ends, said invention then comprises the features hereinafter fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principle of the invention may be employed.

In the practice of my invention, I place a soluble, chemically reducible compound of the metal to be plated (e.g., stannous chloride), a soluble chemical-1y reducible compound of mercury (e.g., mercuric chloride), and a suitable reducing agent (e.g., aluminum powder), all in dry form, directly on the metal surface to be plated. If desired, an inert extender may be included. Then I apply a common solvent for the chemically-reducible compounds, ordinarily water, preferably accompanied with a scrubbing action as with a brush. An amalgam of the plating metal is formed which is bonded to the metal surface, preferably by being alloyed therewith. A continuous, highly tenacious coating of substantial thickness comprising the amalgam results.

The results obtained in accordance with the present invention are markedly improved over those resulting when the mercury compound is omitted. The solid amalgam formed alloys well with the metal to be plated, forming a continuous plating which is bonded metallurgically to the metal surface being plated. I have found that, for example, an amalgam plating of tin and mercury is applied much more quickly than before on copper, bonze, and steel.

The brazing action on such a plating is even better than when hot dip tin is used. Also, articles plated in accordance with the present invention have a surface which is less porous.

While I do not wish to be limited by any theory regarding the manner in which the above operation takes place, nevertheless I believe that it may properly be explained as follows. In the case of stannous chloride, mercuric chloride, and aluminum powder, for example, the finely divided powder co-reduces the stannous and mercuric chlorides as soon as the latter dissolve, which may be very promptly, to release mercury and tin metal while forming aluminum chloride. Inasmuch as the mercury and tin are released in high concentration directly at the surface of the article to be plated, and in highly active or nascent form, such elements quickly form an amalgam which alloys itself with the surface on which the co-reduction takes place to form a coating layer rather analogous to that obtained in the conventional hot dip process. This interpretation is borne out by photomicroscopic inspection which appears to show a tin alloy layer directly bonded to the underlying base metal and which may also have an outer layer of metallic tin. A section through the coating rather closely resembles in appearance a section through a coating produced by the conventional hot dip process with respect to the type of bonding present. It is possible, however, that instead of simple alloying of the tin amalgam with the underlying base metal, an intermetallic compound may be formed, at least in some cases.

It is also postulated that while moistening the powder as with water starts the co-reduction of the mercury compound and the alloying metal compound, the pressure formed by rubbing the moistened powder over the surface of the article to be plated produces plastic deformation although probably only a few molecules deep. The heat of the reaction then produces recrystallization across the boundary, resulting in the formation of new crystals which are an integral part of both the article being plated and the tin-mercury alloy plating. This has been confirmed by a microscopic examination of samples.

In contrast to the foregoing, if a solution of the stannous and mercuric chlorides is first prepared, and then the aluminum powder added and a metal object to be plated is immersed in the solution, the described plating action does not take place.

In addition to stannous chloride and aluminum, I have found that this method of plating an amalgam is applicable to a number of chemically reducible salts and reducing agents, metals on which such plating may be performed, and solvents which may be employed. To illustrate, plat ing metals may include tin, nickel, cadmium, chromium, silver, copper, lead, aluminum, and zinc. Any compound of these metals which is soluble in a solvent inert with respect to the ingredients may be used. Generally, the salts of the inorganic acids are used such as the chlorides, sulfates, nitrates, acetates, and the like. The metal salt used should be in a reducible form, that is, salts such as stannous chloride, nickelous nitrate, chromous sulfate, and the like. It is possible to alloy an amalgam of a metal on an article of the same metal by the present invention.

In a like manner, any reducible compound of mercury, usually a salt, which is soluble in the solvent selected may be used. Where water forms the solvent as is usually the case, mercuric compounds such as mercuric acetate, mercuric bromate, mercuric bromide, mercuric carbonate, mercuric chlorate, mercuric silico-fiuoride, mercuric fluotn'de, mercuric sulfate, and mercuric nitrate may be used. Mercurous compounds may also be used although they are normally less water-soluble than the corresponding mercuric compounds. For example, mercurous acewith the mercury, it is the metallic cation or moiety of the compound that is useful in accordance with the present invention, the balance of such compound being washed, rubbed away, or otherwise removed and forming no important part or function in practicing the present invention.

Since the function of the solvent is merely to dissolve the compounds of mercury and the amalgamating metal, any liquid which performs this ofiice and is chemically inert with respect to the other ingredients may be used. It is merely necessary to bring such compounds, usually salts, into the solution for intimate reactive contact with their common reducing agent. This reaction is exothermic in character and may cause the applied solvent to boil, but the heat developed would not appear to be sufficient in itself to account for the superior bond achieved between the plating amalgam and the metal surface which is plated. As indicated, water is usually the solvent employed, although methyl and ethyl alcohols as well as other organic solvents which mutually dissolve the mercury and alloying metal compounds are within the contemplation of the present invention.

Similarly, the reducing agent may be any metal capable of releasing, as by co-reduction the mercury and alloying metals from their compounds. Ordinarily, aluminum, magnesium, or Zinc is employed for this purpose. When aluminum as in aluminum chloride constitutes one of the plating metals, the reducing agent may be magnesium. When zinc as in zinc sulfate constitutes one of the plating metals, the reducing agent may be aluminum. In still other instances, as in the case of a lead salt like lead acetate, the reducing agent may also be either tin or iron in addition to aluminum, magnesium, or zinc. As an example, the average mesh size employed for the reducing agent powder is 100 percent through 80 mesh and 80 percent through 325 mesh.

As indicated, the plating is usually made on a surface of iron or steel, but plating in accordance with the present invention can also be made on copper and the various brasses or bronzes. The alloying of the amalgam onto a metal base is very strong, and the coating of the metal so plated cannot be rubbed off.

The concentration of the reducible salts in the solution that is formed does not appear to be at all critical, since the reaction, whatever its nature may be, proceeds with whatever amount is present. In actual practice I prefer to form a fairly saturated solution of the mercury and alloying metal compounds and avoid the presence of an undue amount of excess solvent.

The use of an extender assists in preventing any tendency toward premature reaction between the reducible metal compounds and the reducing agent when all of the ingredients are mixed in a container. The extender may also assist in application of the material when such mixture has been sprinkled on the surface to be coated, wetted, and scrubbed with a brush. A preferred extender is diatomaceous earth, but other extenders such as clays, powdered glass, and ceramics like aluminum oxide may be used.

In practice, the metal salts (including the mercury salt) and reducing agent are used in a weight ratio of about 1.5:1 to 2.5:1. When an inert extender is used, the composition may vary approximately as follows:

Percent Reducible metal compound From to 50 Reducible mercury compound From 1 to Reducing agent From 3 to 15 Extender From to 86 A preferred composition for practicing the invention is:

Stannous chloride 20% (approx). Mercuric chloride 5% (approx). Aluminum powder 10% (approx). Celite (diatomaceous earth) 55% (approx).

In order to demonstrate the invention, the following examples are set forth for the purpose of illustration only. Any specific enumeration or detail mentioned should not be interpreted as a limitation of the invention unless specified as such in one or more of the appended claims and then only in such claim or claims.

Example I The metal surface to be plated should be properly cleaned prior to application of the plating material as by wire brushing or the like. A mixture of stannous chloride plus mercuric nitrate and aluminum powder in a weight ratio of 2:1, that is salts to powder, was sprinkled on a cast iron plate. A small amount of methyl alcohol was then applied to the plate sufficient to dissolve the metal salts while the mixture was simultaneously rubbed against the plate with a brush.

An amalgamated tin coating of excellent quality was obtained having a thickness of about one-thousandth of an inch comprising a base layer which appeared to be alloyed with the underlying metal, and an outer more porous or granular layer of pure tin which could be burnished in the usual manner, if desired. The coating was highly satisfactory for brazing purposes, corrosion resistance, etc. As indicated above, such coating appears to be rather analogous to that obtained by hot dip tin plating rather than that obtained by electroplating where no alloy layer is achieved. The coating does not tend to flake or peel but has a very tenacious bond.

Example [I A procedure was carried out similar to the procedure of Example I except that a mixture consisting by weight of 12 percent nickel sulfate, 5 percent mercuric nitrate, 4 percent magnesium powder, and percent powdered glass was used. These materials were sequentially placed upon the cast iron plate and then mixed prior to the addition of the solvent, which in this case was water. A good quality nickel-mercury coat was obtained on the cast iron plate.

Example III A procedure was carried out similar to the procedure of Example I except that a copper plate was used, and the mixture consisted by weight of 20 percent cadmium chloride, 4 percent mercuric acetate, 8 percent zinc powder, and 72 percent finely ground aluminum oxide. The solvent was water. An excellent cadmium-mercury plate was obtained.

Example IV A procedure was carried out similar to that of Example I except that a bronze plate was used and the mixture consisted by weight of 40 percent silver nitrate, 13 percent mercuric chloride, 15 percent aluminum powder, and the balance clay. The solvent was ethyl alcohol, and the mixture was rubbed against the bronze plate after application of the solvent. The heat of the resulting exothermic reaction in time evaporated the alcohol. An excellent silver-mercury plate was produced.

It will now be apparent that I have provided a method of plating metals with an amalgam which may be conveniently accomplished in the field. My method does not require the application of external heat or electricity or chemical displacement of part of the metal to be plated. The resulting surface coating produced by the present method has substantial thickness and impermeability. In the case of plating metals like tin, the surface coating is capable of being burnished.

Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims or the equivalent of such be employed.

I therefore particularly point out and distinctly claim as my invention:

1. The method of plating a mercury alloy on a metal surface which comprises applying a chemically reducible salt of both mercury and a metal to be alloyed therewith and a powdered reducing agent for such salts, all in dry form, directly on such metal surface, dissolving such salts in a common solvent therefor with consequent action by such reducing agent to reduce both salts and produce the plating metals in active form directly at the surface to be plated, whereby such plating metals are immediately alloyed and bonded to such metal surface to form a tenacious coating.

2. The method as defined in claim 1 wherein the chemically reducible salt of the metal to be alloyed with mercury is a salt of a metal selected from the group consisting of tin, nickel, cadmium, chromium, silver, copper, lead, aluminum, and Zinc.

3. The method as defined in claim 1 wherein such metal surface to be plated is of a metal composition selected from the group consisting of copper, steel, iron, brass, and bronze.

4. The method as defined in claim 1 wherein the chemically reducible salt of both mercury and the alloying metal is a salt of an inorganic acid.

5. The method as defined in claim 1 wherein the chemically reducible salt of both mercury and the alloying metal is a salt of an acid selected from the group consisting of hydrochloric, sulfuric, nitric, and acetic acids.

6. A method as defined in claim 1 wherein such reducing agent is a metal differing from the plating metals and selected from the group consisting of aluminum, magnesium, Zinc, tin, and iron.

7. A method as defined in claim 1 wherein the total of such reducible salts and the reducing agent are present in a weight ratio within the range of about 1.5 :1 to 2.5 1, respectively.

8. A method as defined in claim 1 wherein the solvent for dissolving such reducible salts is selected from the group consisting of water, methyl alcohol, and ethyl alcohol.

9. The method of plating a mercury amalgam on a metal which comprises forming a dry mixture on a surface of such metal comprising a chemically reducible salt of both mercury and an amalgamating metal; a powdered reducing agent for such salts; and an extender chemically inert to such salts, agent, and metal surface; dissolving the salts in a solvent therefor with consequent action by such powdered reducing agent to co-reduee such salts and produce the amalgamating metals in high concentration in active substantially nascent form directly at such surface to be plated, whereby due to such nascent form such metals immediately amalgamate and the resulting amalgam alloys itself with such surface to form a continuous tenacious coating.

10. The method as defined in claim 9 wherein such extender is an inert material selected from the group consisting of diatomaceous earth, clay, powdered glass, and powdered aluminum oxide.

11. The method as defined in claim 9 wherein such mixture consists by weight of from about percent to about 50 percent of the reducible salt of the amalgamating metal, about one percent to about percent of the reducible salt of mercury, about 3 percent to about 15 percent of the reducing agent, and from about percent to about 86 percent of the extender.

12. The method as defined in claim 9 wherein such metal consists by weight of about 30 percent stannous chloride, about 5 percent mercuric chloride, about 10 percent aluminum powder, and about 55 percent diatomaceous earth.

13. The method of plating an amalgam of mercury and tin on iron which comprises forming a dry mixture on a surface of such iron of about 30 percent stannous chloride, about 5 percent mercuric chloride, about 10 percent aluminum powder, and about 55 percent diatomaceous earth; dissolving the stannous and mercuric chlorides in a common solvent therefor with consequent action by such aluminum powder to co-reduce such chlorides and produce mercury and tin in high concentration in active form directly on the iron surface to be plated while simultaneously forming aluminum chloride effective to prepare a surface of the iron for a plating action; and simultaneously rubbing such mixture against the iron surface, whereby due to such active form the tin and mercury instantly amalgamate with each other and substantially immediately alloy the resulting amalgam with such iron surface to form a continuous tenacious coating of substantial thickness.

14. The method of plating a mercury amalgam on a metal which comprises forming a dry intimate mixture consisting in greatest quantity of a chemically inert extender together with a chemically reducible salt of both mercury and an amalgamating metal, at least one of such salts being the chloride salt, and a sufficient amount of a powdered reducing agent to reduce such salts, applying the mixture in dry condition to a surface of such metal to be plated, adding a sufiiciently small amount of solvent to the mixture on such surface to dissolve at least some of each of such reducible salts without disturbing an intimate association with the reducing agent to co-reduce such salts and produce mercury and the amalgamating metal in high concentration in active substantially nascent form directly at such surface to be plated while simultaneously forming aluminum chloride effective to prepare that surface for a plating action, and then rubbing the mixture against such surface, whereby due to such active nascent form the mercury and amalgamating metal instantly amalgamate and alloy the resulting amalgam to such surface to provide a continuous tenacious coating thereon.

15. A dry mixture for plating a metal with an alloy consisting essentially of approximately the following proportions by Weight:

Reducible metal compound From about 10% to about 50%. Reducible mercury compound From about 1% to about 15%. Reducing agent From about 3% to about 15% Extender From about 20% to about 86%.

16. A dry mixture for plating a metal with an amalgam consisting essentially of approximately the following proportions by weight:

Percent Stannous chloride 30 Mercuric chloride 5 Aluminum powder 10 Diatomaceous earth 55 References Cited in the file of this patent UNITED STATES PATENTS 547,824 Vigneron Oct. 15, 1895 1,161,944 Maddy et al Nov. 30, 1915 2,072,229 Waitman Mar. 2, 1937 2,093,098 Renck Sept. 14, 1937 2,136,496 Epperson Nov. 15, 1938 

1. THE METHOD OF PLATING A MERCURY ALLOY ON A METAL SURFACE WHICH COMPRISES APPLYING A CHEMICALLY REDUCIBLE SALT OF BOTH MERCURY AND A METAL TO BE ALLOYED THEREWITH AND A POWDERED REDUCING AGENT FOR SUCH SALTS, ALL IN DRY FORM, DIRECTLY ON SUCH METAL SURFACE, DISSOLVING SUCH SALTS IN A COMMON SOLVENT THEREFOR WITH CONSEQUENT ACTION BY SUCH REDUCING AGENT TO REDUCE BOTH SALTS AND PRODUCE THE PLATING METALS IN ACTIVE FORM DIRECTLY AT THE SURFACE TO BE PLATED, WHEREBY SUCH PLATING METALS ARE IMMEDIATELY ALLOYED AND BONDED TO SUCH METAL SURFACE TO FORM A TENACIOUS COATING. 